1. The Field of the Invention
This invention pertains to hydrophobic polyolefin fibers, their fabrication, and to nonwoven fabrics made therefrom.
2. The State of the Art
Synthetic, polymeric fibers have found a wide range of applications, from textiles for clothing to reinforcement for tires. The particular application to which the fiber is put will dictate the physical and chemical properties required. Synthetic fibers are particularly useful in absorbent product, especially coverstock fabrics for diapers and other incontinence and hygiene products, such as sanitary napkins, tampons, underpants, and the like. Polyolefin and other fibers used in coverstock and similar fabrics that permit liquid to flow through them are hydrophobic. To facilitate the flow of liquid through them, they generally comprise a hydrophilic finish so that the liquid flows at a sufficiently high rate. The associated portions of such products, such as leg-cuffs, waist bands, and medical barriers, are also used to manage the flow of liquid as barriers rather than as channels. Accordingly, it is desirable for certain fibers used in these associated portions not only to be hydrophobic but also to have a fiber/finish surface that is hydrophobic.
To achieve the desired hydrophobicity, silicone fluids are conventionally added to the fiber surface by using such devices as a sprayer or a roller. Silicone fluids are also conventionally applied as a surface lubricant, thus, application of these fluids to the surface of the fiber provides a lubricated, hydrophobic fiber. When silicone fluids are used as a hydrophobic finish, they must firs be diluted in a solvent to allow for their application to the fiber surface in a controlled manner. In most cases, silicone fluids used on conventional hydrophobic polypropylene fibers are emulsified in an aqueous solution with the aid of wetting agents. One problem encountered with the use of emulsified silicones is a reduction in the hydrophobicity imparted by the silicone to the fiber surface due to the presence of the wetting agents used in the emulsion. Another problem in using topically applied silicone fluids is that a certain amount of necessary friction is lost because of the lubricity of the silicone fluid. Certain typical fiber processing operations, such as crimping and carding, require a minimum degree of friction between the fiber and parts of the processing equipment in order for the apparatus to manipulate the fiber. The topically applied silicone lubricant interferes with the frictional properties required for these operations. To compensate for the reduced friction, such operations must be performed at lower line speeds, and so the entire process must be slowed down to compensate.
Another problem encountered when using applied silicone (hydrophobic) lubricants, which stems from its alteration of the surface properties of the fiber, is that even when a fiber can be processed into staple fibers and crimped and carded into a web, the silicone lubricant interferes with the integrity of the web, allowing the carded staple fibers to slip past each other, and so the web begins to pull apart during processing. To compensate, the processing speed again must be slowed.
Yet another problem occurs when using antistatic finishes, which are typically hydrophilic in nature. These finishes are often applied to the fiber to facilitate handling the fiber during processing. Yet they can reduce the effectiveness of any lubricating finish on the fiber, requiring reapplication of the lubricant.
There is a balance between lubricating the fiber for its journey over and through processing equipment and the friction necessary for such equipment to engage and manipulate the fiber. Typically, silicone fluids are applied to the surface of fibers in very small amounts (&lt;0.3 wt. %) to reduce friction. The control of such small levels of topically-added silicone to achieve a uniform application on the fiber surface is very difficult. Also, a severe reduction in fiber friction (from over-application of silicone) can result in various processing problems, including reduced line speeds. On the other side, if a hydrophilic spin finish is first applied to the fiber in order to avoid problems using small amounts of silicone, even if in combination with a silicone lubricant, then the resulting fiber remains hydrophilic.
Examples of more recent fibers having a lubricant thereon are described by Schmalz in U.S. Pat. No. 4,938,832 and EP 0486158A2 (corresponding to U.S. patent application Ser. No. 07/914,213 filed Jul. 15, 1992, abandoned in favor of continuation application Ser. No. 08/220,465, now U.S. Pat. No. 5,721,048, the disclosures of which are incorporated herein b reference), in which the spun fiber is treated with finishes comprising neutralized phosphoric acid esters and polysiloxane compounds.
Johnson and Theyson, in U.S. Pat. No. 5,403,426 and EP 0 516 412 A2 (the disclosures of which are both incorporated herein by reference), describe a cardable hydrophobic polyolefin-containing fiber made with finish compositions including neutralized phosphoric acid esters and lubricants such as esters, polyesters, glycols, capped glycols, alkoxylated products (such as polyoxyethylene or polyoxypropylene), and highly polar or ionic structures made therewith (such as methyl ethyl ammonium methylsulfate) and other compounds described therein. Optionally, such a finish is used in conjunction with an overfinish comprising a neutralized phosphoric acid and optionally a polysiloxane.
Harrington, in EP 0 557 024 A1 and U.S. patent application Ser. No. 08/016,346, now U.S. Pat. No. 5,545,481 which is a continuation of application Ser. No. 07/835,895, abandoned disclosures of which are all incorporated herein by reference), describes polyolefin fibers and nonwoven products made therefrom where the fibers include in their surface an antistatic composition comprising at least one neutralized C.sub.3-12 alkyl or alkenyl phosphate alkali metal or alkali earth metal salt and a solubilizer, such as glycols, polyglycols, glycol ethers, and neutralized phosphoric ester salts having the general formula (MO.sub.x --(PO)--O(R.sub.1).sub.n R).sub.y, wherein M is an alkali or alkali earth metal or hydrogen, R is a C.sub.16 -C.sub.22 alkyl or alkenyl group, R.sub.1 is ethylene oxide or propylene oxide, and n is 1 to 10, x is 1 to 2, y is 2 to 1, and x+y=3. The finish may also contain a lubricant such as mineral oils, paraffinic waxes, polyglycols, and silicones.
Nohr and MacDonald, in U.S. Pat. No. 4,923,914, describe a fiber or film forming polyolefin composition having a particular polysiloxane additive, these additives are generally hydrophilic. The additive is compatible with the polyolefin at melt extrusion temperatures but is incompatible at temperatures therebelow, and is comprises of two moieties, provided in the same additive or in separate additives, if provided as separate additives, both are incompatible with the polyolefin at all temperatures. The moieties are both alkoxy groups, in one case the groups capping the end of the siloxane chain, and in the other case the groups being pendant from the backbone. As a result of the incompatibility, the additive has a concentration with the fiber that increase from the fiber axis to its surface.
Lovgren et al., in U.S. Pat. No. 4,446,090, describes blending high viscosity silicone fluids of a variety of compositions into a variety of different thermoplastic polymers. The ratio of high viscosity silicone fluid to thermoplastic polymer is within the range of 0.005-200.0. The process is especially useful for flame retardant silicone fluids.
Riffle and Yilgor, in U.S. Pat. No. 4,659,777, describe polysiloxane/polyoxazoline copolymers which, when incorporated into a fiber-forming composition, provides a fiber wettable by both polar and non-polar liquids.
Foster and Metzler, in U.S. Pat. No. 4,535,113, describe an olefin polymer composition containing siloxane additives useful for the production of films. The siloxane includes pendant from its polymer backbone a monovalent organic radical containing at least one ethylene oxide group, a vicinal epoxy group, or an amino group.
Steklenski, in U.S. Pat. No. 4,473,676, describes incorporating a cross-linked silicone polycarbinol into film-forming compositions to make polymer compositions having a low coefficient of friction and useful for protective layers in photographic elements.
Hansen et al., in U.S. Pat. No. 5,456,982, describe incorporating a surface active agent, such as an emulsifier, surfactant, or detergent, into the sheath component of a sheath-and-core type bicomponent fiber to render the fiber hydrophobic.
Silicone additives such as described by Nohr and MacDonald (noted above), which are incompatible with the bulk polymer at ambient temperatures but compatible at spinning temperatures, take advantage of a problem with such additives. Higher molecular weight for such additives render the additive less soluble in polypropylene (and in other polyolefins). However, using a lower molecular weight silicone decreases the thermal stability of the lubricating additive.
Also as noted above, it is very difficult to control the topical application of an applied surface finish having ingredients in amounts on the order of only a few tenths of one percent of the total finish composition. It is thus very difficult to provide a homogeneous finish composition having only about 0.3% of the silicone additive, and it is very difficult to provide a uniform coating of such a finish on a fiber. The use of an insufficient amount of lubricant in the finish can be very disruptive to commercial operations. Also, use of too much silicone (which can be on the order of only one-tenth of one percent) can render the fiber too slippery for processing, especially crimping, at commercial speeds. Further, even if the fibers can be crimped and processed into a non-woven web, the strength of the web can be significantly decreased because silicone oil at the surfaces of the fibers to be consolidated (e.g., heat-bonded) interferes with the bonding of the fibers to each other.